The speed of turbine engines in aircraft is critical and must be determined with great precision. Commonly, the speed is sensed by an electronic pickup which senses the passage of multiple metallic protrusion from the shaft or a gear coupled auxiliary shaft, as a string of pulses which are then electronically reduced to a visual readout of engine speed
The most common type of sensor system in the past has been an electronic oscillator which drives a resonant A.C. circuit in the sensing head at a high R.F. frequency. Passage of metal near the sensing head changes the Q of the inductance thereby changing the resonant point which is detected as a pulse. One will appreciate that the sensing head and circuitry of such a system is complex and susceptible to damage due to shock and vibration. Further, this type of system requires careful shielding and design to reduce the effects which may be introduced by stray fields and the like. The sensing head is also temperature limited by its electronics and therefore must be placed in an environment which is not detrimental to its proper functioning.
A second type of sensor utilizing eddy current principles is used, however, it is provided with a single pickup coil which makes it susceptible to stray noise pickup which interferes with the signals being produced by the shaft protrusions. This type of unit does have the advantage of having a rugged sensing head which can be placed within the engine utilizing the turbine for blades which are mounted directly on the shaft as the conductive protrusions for the sensing of speed. The presence of noise requires more complex circuitry to filter the signal to obtain the intelligence required.
The eddy current sensor revealed in the following specification represents an improvement in the art by introducing dual coil differential voltage construction which cancels most of the common mode noise voltages due to stray fields, vibration, shock, etc.
Also because of the very severe environments within a turbine engine in which this device is to be used, the physical design of the unit is critical. The very high temperatures and high vibration levels require a unit which is very rigid and can provide the stability and support for proper sensor operation.